Recent developments in design and fabrication technologies have made it possible to build a large number of micro devices (e.g., electronic devices, mechanical devices, and optical devices) on a single substrate to form a highly integrated micro-system. One or more micro-systems may be further packaged to form a microelectronic module.
While many devices can be successfully miniaturized and integrated, such is not the case with some devices. For example, it is usually difficult to fabricate passive devices (e.g., capacitors and inductors) together with active devices (e.g., transistors). It is even more difficult to scale down passive devices and active devices simultaneously. On the other hand, as modern IC's are operating at faster speeds and lower power levels, it is often necessary to use passive devices, such as decoupling capacitors, to mitigate electro-magnetic interference (EMI) (e.g., cross-talk and power distribution noises) among densely packed circuit elements. However, integrated capacitors and inductors usually take up a substantial amount of chip area and cost extra effort to design or fabricate.
Various approaches have been proposed to implement off-chip passive devices for a microelectronic module. In one approach, passive devices desirable for a microelectronic module are surface-mounted on a circuit board that hosts the microelectronic module. The passive devices are individually packaged elements and are electrically coupled with the microelectronic module via a number of traces. If the microelectronic module has a large number of input/output (I/O) pins or if the I/O pins have small pitches, the passive devices may have to be placed inches away from where they are needed and the traces may run even longer distances. Such remotely placed passive devices can introduce further interferences or other complications to the performance of the microelectronic module. Although passive devices of smaller-than-usual sizes have been tried, they often pose reliability problems and have not been widely adopted.
According to another approach, passive devices desirable for a microelectronic module may be embedded in a circuit board that hosts the microelectronic module. However, circuit boards with embedded passive devices are usually expensive to manufacture. Since the passive devices, once embedded, cannot be altered or replaced, there is no room for error in the design and fabrication of the circuit board. That is, if a finished circuit board with embedded passive devices is found unsatisfactory, the whole circuit board is wasted.
In view of the foregoing, it would be desirable to provide a technique for enhancing circuit density and performance which overcomes the above-described inadequacies and shortcomings.